A small, weather-beaten $10 diamond found in the gravel at the bottom of a shallow river in Mato Grosso, Brazil, has provided evidence of a vast “hidden ocean” deep inside the Earth which could hold as much water as all the world’s seas put together.

In a paper published in the journal Nature March 12, Canadian geochemist Graham Pearson, from the University of Alberta, and his co-authors describe how the gem formed at least 400 kilometres down has yielded the only surface sample ever found of a deep-Earth mineral, providing the first concrete evidence for a long-held theory about how the planet works.

Researchers have long suggested that minerals such as ringwoodite, thought to be one of the main constituents of the Earth’s mantle, have the ability to store water molecules locked within their crystal structure. Pearson and his team, for the first time ever, had a chance to look inside an actual sample.

The water now known to exist deep in our planet may help scientists better understand major geological processes.